In gas turbine engines, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which turns the shaft and provides power to the compressor. The hotter the turbine gases, the more efficient the operation of the engine. Thus, there is an incentive to raise the turbine operating temperature. However, the maximum temperature of the turbine gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine.
A protective layer is applied to the airfoil of the turbine blade or turbine vane component, which acts as a substrate. Among the currently known diffusional protective layers are aluminide and platinum aluminide layers. The protective layer protects the substrate against environmental damage from the hot, highly corrosive combustion gases. This protective layer, with no overlying ceramic layer, is useful in intermediate-temperature applications. For higher temperature applications, a ceramic thermal barrier coating layer may be applied overlying the protective layer, to form a thermal barrier coating (TBC) system. The ceramic thermal barrier coating layer insulates the component from the exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the substrate.
Even with the use of these protective techniques, there remain problems to overcome in certain operating service conditions, particularly within marine turbine engines that are exposed to harsh conditions related to the salinity of the operating environments.